## Modulus function
-$$|x|=\sqrt{x^2}$$
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+$$|x|=\sqrt{x^2}$$
+
+## Parametric equations
+
+### Circles
+$$\[\begin{cases}
+ x=a\cos t\\
+ y=a\sin t
+ \end{cases}
+\text{where radius} =a$$
+
+To convert to cartesian, factorise and use $\cos^2 x + \sin^2 x=1$
+
+$\cos^2 t + \sin^2 t = 1$
+$\implies {\cos^2 \over \sin^2 t} + {\sin^2 t \over sin^2 t} = {1 \over \sin^2 t} \implies \csc^2 t - \cot^2 t$